The test-retest reliability of a survey instrument, like a psychological test, is estimated by performing the same survey with the same respondents at different moments of time. The closer the results, the greater the test-retest reliability of the survey instrument. The correlation coefficient between such two sets of responses is often used as a quantitative measure of the test-retest reliability.
For example, a group of respondents is tested for IQ scores: each respondent is tested twice - the two tests are, say, a month apart. Then, the correlation coefficient between two sets of IQ-scores is a reasonable measure of the test-retest reliability of this test. In the ideal case, both scores coincide for each respondent and, hence, the correlation coefficient is 1.0. In reality, this is almost never the case - the scores produced by a respondent would vary if the test were carried out several times. Normally, values of the correlation 0.7...0.8 are considered as satisfactory or good.
The test-retest reliability is the most popular indicator of survey reliability.
A shortcoming of the test-retest reliability is that the "practice effect" - respondents "learn" to answer the same questions in the first test and this affects their responses in the next test. For example, the IQ-scores may tend to be higher in the next test.